"How do you fill it?"Seriously? There's a threaded cap completely visible in that photo.

The water is in the 2" half, while the air is in the 3" half... It was just a throw together, as I already had a 3" chamber, not in use...

I'm going to place it in a large backpack after I get some hoseclamps.

You hold it straight up\down starman, The water needs to be at the bottom of the "lower tank" in the image, so that it flows out the hose.

At first, it just "peed" water at 20 PSIG... Then I put on the Rubber Blowgun Tip... Shot like a thumb over a water hose... Up to 60 PSIG, it has a lot of force without the nozzle... With the nozzle, you've got a pressure washer... That was without hoseclamps, so I had a few people holding the tube together... with some leaks